Difficult topic... Agh, it hurts to think back ten whole years!
So... 2009: Demon's Souls.
Look, I know people didn't really enjoy it because of a few reasons: The difficulty, the difficulty... the, um... Difficulty? Oh! The lack of an expansive inventory of items and such. Yes... I know there are about two dozen "Full Sets" to collect, but seriously, you get the best "Tank" armor after playing the game for two hours. After that it's pretty hard to fall back on any other character build as being a "Tank" works oh, so well! But, I liked the game. It took me forever to collect everything (as worthless as most of it was), and the bosses were repeatedly satisfactory.
But the Decade? Game of the decade... I gotta say it's a close tie.
My first choice would be Gears of War 2. I can't recall how many HOURS of multiplayer I have logged on that game with seven other guys I work with. DAYS worth of blissful carnage.
My second choice would be Fallout 3. Not since Oblivion has any game grabbed me for so long, with so much to do and with such fun. I sincerely enjoyed Fallout 3 and its expansions. Made me go back a little and play Fallout 1 & 2.
-If re-releases count here... Final Fantasy 7 & 8 on PSN. Ah, memories.-